1. Field of the Invention
The present invention relates to a method for measuring the shape of an article, and more particularly a method for measuring the shape of a glass sheet.
2. Technical Background
Thin sheets of glass are finding use in an increasing array of manufactured articles, and in particular electronic articles including both desktop and laptop computers, cell phones and televisions. Television displays, especially, are undergoing a transformation from historically old cathode ray tubes (CRTs) to plasma and liquid crystal displays (LCDs). Additionally, consumer demand is pressing manufacturers of such non-CRT displays to offer increasingly larger display sizes.
The need to produce larger and larger sheets of display glass while adhering to already stringent requirements pertaining to such parameters as surface quality are pushing the limits of existing methods of manufacturing glass sheet. It is known, for example, that residual stress which may exist within a large glass sheet will cause a smaller sheet which may be cut from the larger sheet to assume a shape different than the parent glass as those pre-existing stresses are relieved and/or redistributed.
Glass sheet may be formed by a variety of methods, including the well-known float process, wherein a glass melt is floated on a bath of liquid metal, typically tin. Another popular method of manufacturing glass sheets is known as the fusion draw method (FDM), wherein a molten glass is fed into a trough. The molten glass overflows both sides of the trough in a controlled manner, and the separate glass flows are re-united at the apex of the trough. Accordingly, the exposed surface of the glass sheet is pristine and the glass sheet may be drawn from the apparatus in a ribbon. A fusion process for forming glass sheet is explained more fully in U.S. Pat. Nos. 3,338,696 and 3,682,609 (Dockerty), the contents of which are incorporated herein in their entirety by reference.
When forming thin glass sheet by a continuous ribbon forming method such as the overflow downdraw, or fusion process, shape can be imparted on the sheet product. Sheet shape can manifest itself in many forms, including bow, sag, “s-warp”, etc. Ultimately, the sheet shape can become a problem for downstream customers for several reasons: edges of the glass that are not in a plane can become a source of breakage due to impact; severely shaped glass may not vacuum chuck down in the various tools used to manufacture LCDs thereby leading to throughput slowdown or may generate high stress levels in the glass substrate leading to breakage. Even moderately shaped glass may not lay down on various chucking devices, thus leading to non-uniformities in the deposited thin films which make up the electronic portions of the display. In order to make a low stress and/or low warp product it is required that sheet shape be fundamentally understood and reduced to minimal levels.
A non-planar sheet shape can be caused by a number of processing factors, including bending and vibration of the drawn glass ribbon within the elastic temperature range which is transmitted upward into the visco-elastic region, and “frozen-in” thermal stress effects. Such movement may result from cutting of the ribbon into a separate pane or sheet. Shape may also result from frozen-in stresses, such as can occur when a non-flat, across-the-draw temperature gradient occurs in the sheet as it passes through the visco-elastic temperature range. Because in many cases the drawn glass ribbon is exceptionally thin (such as glass used in display devices) much of the frozen-in stress may be compensated for by shape formation. That is, the ribbon deforms to relieve the stress. This form of shape is transient in nature, and may be substantially relieved or redistributed upon cutting of the ribbon into individual sheets, or later re-cutting of the sheet into still smaller portions.
While sheet shape has been examined to some extent, precise sheet shape measurement methods are required to better analyze the contour of the sheets, in order to best control the formation of shape. In addition, while current mechanical feeler gauge methods can analyze sheet shape to some degree, it is difficult to eliminate gravity-based sag effects from the measurement.